bpo-38834: enable runtime inspection of TypeDict classes

[Zac-HD]

This patch enables downstream projects inspecting a TypedDict subclass at runtime to tell which keys are optional, which is essential for generating test data with Hypothesis or validating inputs with typeguard or pydantic.

More background on the typing-sig mailing list (cc @ilevkivskyi).

https://bugs.python.org/issue38834

[ilevkivskyi]

Thanks! All looks good.

[ilevkivskyi]

@gvanrossum What do you think, can we include this in 3.8.1? Or should it just go to master (3.9) and typing_extensions?

[gvanrossum]

@gvanrossum What do you think, can we include this in 3.8.1? Or should it just go to master (3.9) and typing_extensions?

I'd put in 3.9 (master) and in typing_extensions.

[ilevkivskyi]

OK, @Zac-HD could you please make a PR to https://github.com/python/typing adding this to typing_extensions?

[agronholm]

@Zac-HD I don't think this was intended:

>>> DummyDict = TypedDict('DummyDict', {'x': int, 'y': str}, total=False)
>>> DummyDict.__required_keys__
frozenset({'x', 'y'})

__required_keys__ turns out the same regardless of whether total is False or True.
It's unfortunate that this got in the final release.

[agronholm]

The bug is not really your fault though; It happens because def TypedDict(typename, fields=None, /, *, total=True, **kwargs): does not pass total to _TypedDictMeta(). It still passes __total__ in the attribute namespace, but your code does not check that. A test against the older syntax should have been included.

[agronholm]

Created https://bugs.python.org/issue42059.